The present invention relates to estimation of a signal to interference ratio (SIR) in a mobile communication system, particularly where an RF signal is received at a second station along a plurality of multi-paths from a first station.
The invention is particularly but not exclusively concerned with a WCDMA communication system (wide band code division multiplexed) in which an information signal is encoded for transmission by modulating data symbols to be transmitted using a unique spreading code for each channel. Spreading codes transmitted on the downlink from a base station are preferably orthogonal to reduce interference between signals sent to a plurality of different mobile stations.
This has the effect that a signal received at any particular mobile station contains not only the information destined for that mobile station itself, but also interference constituted by signals destined for other mobile stations in the network. This interference can include communication channels I0R within the same cell as the mobile station in question, but can also include interference from outside that cell I0C. Moreover, in travelling from the base station to the mobile station, the signal may well have travelled along a plurality of multi-paths depending on the environment in which the cellular communication system is located. That is, the multi-paths are dependent on obstructions, reflections etc between the base station and the relevant mobile station. In a known wideband CDMA terminal, the so-called wideband signal (that is the signal transmitted between the base station and the mobile station including a number of communication channels overlaid one on the other) is supplied to a plurality of rake fingers which each generate a narrowband signal by despreading the information contained in the signal using the unique spreading code for that particular communication channel. A plurality of narrowband signals are generated representing the signal received along each of the plurality of multi-paths. Of course, it is not possible to determine precisely the nature of the multi-paths travelled by the information signal between the base station and the mobile station, so the number of rake fingers is selected according to the particular environment in an attempt to make an estimate of the likely number of paths involved. A synchronisation unit attempts to determine on receipt of the signal the number of paths and the phase difference between the paths to supply this information to each rake finger.
The known wideband CDMA terminal also includes a fast closed loop power control (TPC) which generates a power control bit which is transmitted from the mobile station to the base station to control the transmission power on the downlink so that the downlink signal is received at a level which ensures proper decoding of the information. This fast closed loop power control utilises an SIR estimate to determine how the TPC bit should be set.
The aim of the fast loop power control is to maintain the signal quality, that is the ratio of the required information signal level with respect to noise level, as stable as possible while minimising transmission power of the base station. This requires an accurate estimate of the required information signal level with respect to the interference received from other channels within the same cell or from other cells.
One attempt which has been made to produce an SIR estimate is to use the wideband signal (before despreading) in estimating the interference part of SIR. However, on the downlink this does not take into account the effect of providing a plurality of orthogonal spreading codes within each cell. For example, if there is a single path channel and the mobile terminal is close to the serving base station, the signal to interference ratio is badly underestimated because all the interference from the serving base station is included in the wideband interference estimate, although in fact this is cancelled from the signal after the despreading operation because all orthogonal codes apart from the unique spreading code have been removed from the generated narrowband signals.
According to another attempt, an average narrowband interference is used as the interference estimate. This is an improvement on the use of the wideband signal, but only takes into account the average orthogonality in the case of equally strong multi-paths. In a more realistic situation where the signal power levels received along a plurality of multi-paths are likely to be quite disparate, the SIR is underestimated.
It is an aim of the present invention to provide an improved estimate of SIR which correctly takes into account the use of orthogonal spreading codes in a multi-path environment within a cellular communication system.
According to one aspect of the present invention there is provided a method of estimating a signal to interference ratio in a cellular communication system wherein a signal is transmitted from a first station to a second station along a plurality of different paths, the method comprising: estimating the power level of the signal received along each path; estimating the interference in the signal received along each path; generating a combined power estimate by summing the estimates of power levels received for all the paths; and generating the signal to interference ratio (SIR) as the ratio of the combined power estimate by a combined interference estimate which is the sum over all the paths of the interference estimates weighted by the respective estimated power level for that path divided by the combined power estimate.
Generation of the SIR can include generating the combined interference estimate by weighting the interference estimate for each path by the estimated power level for that path, summing the weighted interference estimates over all of the paths and dividing the resultant sum by the combined power estimate.
In addition to code division multiplexing, the signal can be transmitted in a sequence of time slots as in a TDMA system. In the case where the signal to interference ratio is used at the second station to generate a power control bit for transmission to the first station to control the transmitted power from the first station to the second station, the signal to interference ratio can be calculated in a first time slot, and the power control bit used to control the transmission power on the downlink in a subsequent time slot.
According to another aspect of the present invention there is provided circuitry for estimating a signal to interference ratio in a cellular communication system wherein a signal is transmitted from a first station to a second station along a plurality of different paths, the circuitry comprising: means for estimating the power level and interference of the signal received along each path; means for generating a combined power estimate by summing the estimates of the power levels received for each the paths; and a signal to interference ratio generator for generating the signal to interference ratio (SIR) as the ratio of the combined power estimate by a combined interference estimate which is the sum over all the paths of the interference estimates weighted by the respective estimated power level for that path divided by the combined power estimate.
The signal to interference generator can include means for generating the combined interference estimate by weighting the interference estimate for each path by the estimated power level for that path, summing the weighted interference estimates over all of the paths and dividing the resultant sum by the signal estimate.
According to a further aspect of the invention there is provided a mobile station which comprises circuitry for estimating a signal to interference ratio in a cellular communication system wherein a signal is transmitted from a first station to a second station along a plurality of different paths, the circuitry comprising: means for estimating the power level and interference of the signal received along each path; means for generating a combined power estimate by summing the estimates of the power levels received for each the paths; and a signal to interference ratio generator for generating the signal to interference ratio as the ratio of the combined power estimate by a combined interference estimate which is the sum over all the paths of the interference estimates weighted by the respective estimated power level for that path divided by the combined power estimate.
The embodiment described herein describes the case on the downlink in a mobile communications system. That is, the receive circuitry illustrated in FIG. 3 is located within the mobile station for receiving signals from the base station. However, the technique described herein for SIR estimation could also be used on the uplink, that is on the receive side of the base station.